cosmic string network - kyoto u

Cosmic string network

Yukawa Institute for Theoretical Physics (YITP)Kyoto University

Takashi Hiramatsu

D. Yamauchi (ICRR), C.-M. Yoo(YITP), K. Takahashi(Kumamoto), Y. Sendouda(Hirosaki)

Lunch seminar, 27 Jun 2012 @ YITP

Cosmic String Network

Takashi HiramatsuCosmic strings

1-dimensional topological defect, produced through the spontaneous symmetrybreaking (SSB) of the vacuum state of some kinds of fields.

It's possible for various kinds of field to exist in the early epoch of the universe, and some of them can be responsible for the formation of CSs.

cosmic strings domain wallsour universe(visible region)



● Why CSs have been considered in cosmology ? → originally as the seed of LSS● This role was replaced by inflation after COBE satellite's observation in 1990s which clarified an evidence of acoustic peak in CMB angular power spectrum. That has been manifestly confirmed by WMAP, a following mission of COBE in early 2000s.

● In fact, CS cannot create characteristic peaks in the power spectrum. COBE/WMAP restricts the amount of CS in our universe at most to 10% of the whole energy.

matter

anisotoropy

?

COBE (1989~1993)= COsmic Background ExplorerWMAP (2001~2010)= Wilkinson Microwave Anisotropy Probe

inflation



CSs is still important possible probe for high-energy phenomena in early universe, extending the limit of our scope up to the time just after inflation.

Relation with superstring : macroscopic object of super strings is called cosmic superstring.

Possible source of gravitational waves

Various unobserved physics lying in invisible age

D-string

F-string

Cosmic superstrings Gravitational wave source

inflation

last scattering surface

CM

B?

Invisible age


Takashi HiramatsuScaling property

visible volumebecomes larger

If (comoving) separation of strings is fixed at the SSB, strings dominate the universe.

horizon scale

linear energy density of a string

cf.

According to naive consideration of string evolution, strings are fobidden to survive.

more strings can be seen.


Takashi HiramatsuScaling property

If the self-similarity is maintained, a horizon-sized string exists in a horizon-scale box

horizon scale

visible volumebecomes larger

linear energy density of a string

cf.

Self-similarity of string evolution inner the Horizon can avoid to overclose the universe.

interaction between strings → 'reconnection'reduces the number of visible strings.


Takashi HiramatsuReconnection

reconnection loop production loops radiate GWs and collapse,reducing string energy density

In order to have the scaling property, the reconnection processof strings has to take place efficiently.

GW


Takashi HiramatsuString network simulations

Nambu-Goto simlations (early 1990s ~)

Study the motion of strings by solving the EOM of strings.

No thickness, no interactions between strings.

Reconnection process is artificially imposed, taking place with a given probability.

Enables to perform the quite large scale simulations.

Has a great compatibility with analytic treatments.

Field-theoretic simulations (early 2000s ~)

Solving the field equations of scalar fields coupling with the gauge fields.

Interactions of strings are determined from the potential and the gauge couplings. Reconnection is completely controled by the field interactions.

The scale of simulations is highly restricted by the limitation of computer resources.

Focusing on details of string interactions, small-scale simulations has also been done in a varity of situations.


Takashi HiramatsuField-theoretic : Abelian Higgs model

complex scalar + local U(1) gauge

gauge scalar


Takashi HiramatsuClassification of strings

After SSB, gauge field acquires its mass, and then Scalar mass : Gauge mass :

Classified by

Type-I

Type-II

Critical coupling

Large β limit corresponds to global strings involving axion productioncf. Allen, Kawasaki, Saikawa, Shellard, Sikivie, Yamaguchi, Yokoyama, Yamaguchi, Vilenkin, ...

Most of field-theoretic simulations so far havebeen done wth this condition. cf. Hindmarsh, Bevis, Shellard, Vilenkin, Martin, de Putter, Achucarro, Vachaspati, Davis, ...

Not so well studied in cosmological context.cf. is predicted for stings associated with SSB of flat direction in MSSM

Cui, Martin, Morrissey, Wells, PRD 77 (2008) 043528


Takashi HiramatsuOur studies

Strong coupling with gauge fields doesn't prevent scaling ?

Type-I string has peculiar properties low velocity collision of strings produces a bound state

Small effective reconnection rate ?

Are there any more efficient energy release mechanism than loop production involving the strong gauge coupling ?

Bettencourt et al., PRL 78 (1997) 2066Salmi et al., PRD 77 (2008) 041701R bo

und

stat

e

How does type-I string network evolve ?classical field

Achuccaro, de Putter, PRD 74 (2006) 121701


Takashi HiramatsuNumerical simulation : network

N=512s=18~2

visualised by OpenDXCPU : Opteron 6172*2 (2.1GHz, 24 cores)MEM : DDR3-1333 128GBHDD : 21TB+7TB


Takashi HiramatsuNumerical simulation : colliding strings


Takashi HiramatsuResults : String energy – time evolution

- scaling property is observed at late time- for Type-I, can see heavy oscillations

scaling property

scaling property(?) + oscillation

WORK IN PROGRESS

cosmic string network - kyoto u

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